Nuclear magnetism logging

ABSTRACT

Method and apparatus for analyzing earth materials in situ along a bore hole on the basis of their nuclear magnetic properties. A logging instrument is passed through the bore hole with means to subject a region including earth material in situ to a relatively strong D.C. magnetic field, which may be oriented in any direction and which may be relatively inhomogeneous, for polarizing susceptible nuclei in said material, including selected nuclei therein. The logging instrument also contains means for applying an alternating magnetic field to the same region, including polarized nuclei therein, in a direction transverse to the earth&#39;&#39;s magnetic field and at a frequency corresponding to the Larmor precession frequency of the selected nuclei in the earth&#39;&#39;s field. While the alternating field is being applied, the D.C. polarizing field in the region including said material is terminated within a predetermined time interval, i.e., turned off sufficiently slowly to provide an adiabatic variation of the D.C. field and sufficiently quickly to terminate the D.C. field within a time interval less than the characteristic thermal relaxation time of the selected nuclei. Since the frequency of the alternating field is such that it produces resonance in the earth&#39;&#39;s field alone, turning off the polarizing field sweeps the nuclei onto driven resonance. The alternating field is then abruptly terminated in said region including said polarized nuclei so that the selected polarized nuclei precess freely about the earth&#39;&#39;s relatively homogeneous magnetic field. A characteristic of the resulting free precession condition of the selected nuclei is detected. In accordance with a preferred embodiment, the free precession condition is detected by phase coherent detection utilizing a reference signal produced in timed correlation with the alternating field. Advantageously, in accordance with a further feature of the invention, computer of average transient signal averaging techniques, such as the type known as boxcar integration (digital or analogue), are employed for enhancement of the detected free precession signal with respect to the noise.

United States Patent Slichter [451 May 30, 1972 [54] NUCLEAR MAGNETISM LOGGING [72] Inventor: Charles P. Slichter, Champaign, Ill.

[73] Assignee: Texaco Development Corporation, New

York, NY.

[22] Filed: Mar. 17, 1970 [21] Appl.No.: 20,207

D. C. Ailion and C. P. Slichter0bservation of Ultra-Slow Translational Diffusion in Metallic Lithium by Magnetic Resonace-Phys. Rev. 137( lA)-l/4/65-pp. A235- A245 Primary Examiner-Michael J. Lynch AnarneyThomas l-l. Whaley,'Carl G. Ries and Robert J. Sanders, Jr.

57 ABSTRACT Method and apparatus for analyzing earth materials in situ along a bore hole on the basis of their nuclear magnetic properties. A logging instrument is passed through the bore hole with means to subject a region including earth material in situ to a relatively strong D.C. magnetic field, which may be oriented in any direction and which may be relatively inhomogeneous, for polarizing susceptible nuclei in said material, including selected nuclei therein. The logging instrument also contains means for applying an alternating magnetic field to the same region, including polarized nuclei therein, in a direction transverse to the earths magnetic field and at a frequency corresponding to the Larmor precession frequency of the selected nuclei in the earths field. While the alternating field .is being applied, the DC. polarizing field in the region ineluding said material is terminated within a predetermined time interval, i.e., turned off sufficiently slowly to provide an adiabatic variation of the DC. field and sufficiently quickly to tenninate the DC. field within a time interval less than the characteristic thermal relaxation time of the selected nuclei. Since the frequency of the alternating field is such that it produces resonance in the earths field alone, turning off the polarizing field sweeps the nuclei onto driven resonance. The alternating field is then abruptly terminated in said region including said polarized nuclei so that the selected polarized nuclei precess freely about the earths relatively homogeneous magnetic field. A characteristic of the resulting free precession condition of the selected nuclei is detected. In accordance with a preferred embodiment, the free precession condition is detected by phase coherent detection utilizing a reference signal produced in timed correlation with the alternating field. Advantageously, in accordance with a further feature of the invention, computer of average transient signal averaging techniques, such as the type known as boxcar integration (digital or analogue), are employed for enhancement of the detected free precession signal with respect to the noise.

28 Claims, 61 Drawing Figures PATENTEnmao I972 3,667, 035

sum 01 HF 12 PATENTED AY SHEET 02 [IF I2 PATENTEDmao I972 3, 667, 035

sum nanr12 PATENTEDIAY 30 I972 sum 09 0F 12 Ncyr/e: 0/

sum 10 0F 12 PATENTEDHAY 30 I972 PATENTEDMY 30 :972

sum 11 0F 12 UUUUU NUCLEAR MAGNETISM LOGGING This invention relates to nuclear magnetism well logging of earth formations in situ along a bore hold; and, more particularly, is related to improvements in nuclear magnetic resonance well logging techniques involving the making of such measurements utilizing the earths magnetic field.

Methods and apparatus have been proposed heretofore for conducting magnetic resonance logging operations along an earth bore hole to analyze the constituents of material comprising earth formations in situ. One such proposal involves so-called forced precession techniques using a logging instrument containing means for subjecting the earth formations to a relatively strong D.C. polarizing magnetic field and a transverse RF alternating magnetic field at the Lannor precession frequency of selected polarized nuclei to cause such nuclei to precess about the DC. field so that the appropriate magnetic resonance condition may be produced in the formation where selected nuclei having requisite magnetic properties may be present. The resultant magnetic resonance condition of the sample is then detected by observing the energy absorbed from the RF coil used to produce the transverse field or by utilizing a separate detector coil for measuring the condition of magnetic resonance. A problem associated with such socalled forced precession techniques is that they require large magnets to provide relatively homogeneous D.C. fields in which the magnetic resonance condition is observed.

Since it is desirable to conduct magnetic resonance analysis in the presence of a relatively homogeneous D.C. magnetic field, it has been proposed to conduct magnetic resonance well logs using techniques for causing selected polarized nuclei to precess about the earths magnetic field. In accordance with these techniques, selected nuclei under investigation are polarized, usually by the application of a pulsed unidirectional magnetic field which is substantially perpendicular to the earths magnetic field and is turned off rapidly, rather than adiabatically, whereby the nuclei are caused to precess about the earths field at the Larmor frequency. This requires that the condition of resonance be initiated in a relatively weak field, e.g., the earths field.

These measurements may involve free induction techniques or other techniques suitable for measuring the magnetic resonance condition of a sample containing nuclei with the requisite magnetic characteristics. In this respect, for example, see US. Pats. Nos. 3,135,912, 3,213,354, 3,213,356, 3,213,357 and 3,226,632.

Among disadvantages of such prior techniques using the earths magnetic field are l that the requirement to turn the polarizing field off quickly limits the strength of the polarizing field and thus the signal strength, and 2) that they result in relatively weak signals relative to noise, which signals, if rectified, do not lend themselves to the use of signal averaging techniques of signal enhancement.

The present invention is directed toward improvements in magnetic resonance well logging featuring advantages found in the so-called forced precession techniques and which, nevertheless, utilizes the relatively homogeneous earths magnetic field in which to make the desired magnetic resonance measurement. Since the polarizing field can be turned off relatively slowly, a stronger polarizing field can be utilized. Embodiments of this invention also involve the use of improved signal detection and enhancement techniques for obtaining improvements in signal to noise ratios.

SUMMARY OF THE INVENTION Briefly stated, one aspect of the present invention involves method and apparatus for analyzing materials in situ in the earth along a bore hole on the basis of their nuclear magnetic properties. A region of the earth including material in situ is subjected to a relatively strong D.C. magnetic field for polarizing susceptible nuclei, including selected nuclei which may be present in said material. An alternating magnetic field is ap plied to the same region including said material in a direction transverse to the earth's magnetic field and at a frequency corresponding to the Larmor precession frequency of the selected nuclei in the earths field. Prior to termination of the alternating field, the polarizing field in the region including said material is terminated adiabatically within a predetermined time interval less than the characteristic thermal relaxation time of the selected nuclei, thereby sweeping onto the condition of driven resonance for selected nuclei in the earth s field. The alternating field is then abruptly terminated in said region leaving the selected polarized nuclei precessing freely about the earth's field. A signal characteristic of the resulting free precession condition of the selected nuclei is detected. Preferably, in accordance with a further embodiment, the alternating field is correlated with a source of oscillations employed to generate a reference signal for phase coherent detection of the free precession signal, and signal averaging techniques may also be used to achieve enhancement of the signal to noise ratio of the detected free precession signal.

The features of the invention which are believed to be novel are pointed out in the appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS In the drawings:

FIG. 1 is a side elevational view, partly in cross section, showing schematically a portion of a borehole in the earth and a logging system including an instrument suspended in the borehole for conducting a well log of the material comprising the earth formations along the borehole in accordance with principles of the invention;

FIG. 2 is a side elevational view showing an enlarged, crosssectional view of the logging instrument illustrated in FIG. 1;

FIG. 3 is a schematic representation, partly in block form, showing in further detail the surface electronics of the logging system of FIG. 1 and further illustrated schematically the circuitry of the instrument of FIGS. 1 and 2;

FIG. 3a is a schematic representation, partly in block form, showingin detail similar to that of FIG. 3 an alternate embodiment of the surface electronics for the logging system of FIG. 1;

FIGS. 4a through 4e are plot vs. time of various aspects of the magnetic fields and signals involved in the carrying out of magnetic resonance analysis according to the present invention;

FIGS. 50 through 5h are further plots vs. time of various aspects of the magnetic fields and signals involved in the carrying out of magnetic resonance analysis according to the present invention, with FIG. 5] being an enlarged detailed illustration of a portion of FIG. 5f showing the characteristic shape of the curve M which is typical of the shape of curves M through M FIGS. 6a through 61' are still further plots vs. time of various aspects of the magnetic fields and signals involved in carrying out magnetic resonance analysis for measuring T (low field) according to the present invention;

FIGS. 6'a through 6'i are still further plots vs. time, similar to FIG. 6a through 61, illustrating various aspects of the magnetic fields and signals for measuring T (low field) in accordance with the invention in the limiting case of H equals 0, i.e., in the earths field.

FIG. 7a is a schematic representation, partly in block form, showing another embodiment of the invention illustrated in FIG. 3;

FIG. 7b is a schematic representation, in block form, showing in further detail the portion of FIG. 7a designated NMR frequency controlling circuit 50.

FIG. 7c is a plot vs. time showing the wave form of the detector output of the apparatus illustrated in FIG. 7a;

FIGS. 8:: through 8d are plots vs. time illustrating various aspects of the signal, noise and reference components related to the phase-coherent detection and box-car integration system illustrated in FIGS. 3 and 7a;

FIG. 9a is a side elevational view, partly in block form, showing a further embodiment of the apparatus illustrated in FIG. 3, suitable for use as a NMR magnetometer; and,

FIGS. 9b through 9f are plots vs. time illustrating the wave form of nuclear magnetism signals provided by the apparatus of FIG. 9a.

DETAILED DESCRIPTION OF THE INVENTION Apparatus in accordance with the invention is shown in FIG. 1 adjacent a borehole traversing a plurality of earth formations ll, 12, 13 about which analytical information is desired. Within the borehole there is shown a logging instrument l4 suspended by means of a cable which also serves to provide a plurality of electrical circuit paths between the instrument 14 and electrical equipment at the u'face of the earth adjacent the borehole, shown schematically as surface electronics l6 and recorder 17. Positioning means for raising and loweringthe logging instrument 14 by means of the cable 15 is shown schematically as a sheave 18, it being understood that the positioning means includes well known equipment such as a drum (not shown) for reeling the cable and a winch (not shown) for raising and lowering the cable 15 as desired. The positioning means further includes means for generating a signal indicative of the amount of cable 15 paid out into the borehole, therefore showing the depth position of the logging instrument 14 in the borehole at various times during a well log. The signal indicative of the position of the logging instrument, which may be derived from a selsyn or the like in known manner,-is shown coupled by means of a suitable electrical connection 19 to the recorder 17 so that a continuous record may be made of theposition of the logging instrument 14 in correlation with logging signals coupled from the surface electronics 16 to the recorder 17.

The logging instrument 14 operates in conjunction with the surface electronics 16 and the recorder 17 to provide signals indicative of certain nuclear magnetic properties of material comprising the earth formations ll, l2, 13 along the traverse of the borehole.

The details of the logging instrument 14 may be better appreciated by reference to FIG. 2 wherein the instrument 14 is shown to comprise an outer shell or housing 20 containing apparatus for producing the desired magnetic fields required to produce the condition of nuclear magnetic precession in selected nuclei adjacent the borehole as well as means for detecting. a condition of nuclear magnetic precession thus produced in the material adjacent the borehole.

. It is to be understood that the housing 20 of the logging instrument 14 may be formed of any material suitable to provide the desired mechanical and structural integrity under the conditions encountered in earth boreholes to be logged, without unduly interfering with the magnetic fields involved in producing and detecting the desired condition of magnetic precession in the formations adjacent the borehole. For example, the housing may be formed of aluminum.

, The logging apparatus within the housing 20 includes means for producing a relatively strong D.C. magnetic field H in the region adjacent the logging instrument 14, shown schematically as an electro-magnet formed of two separate axially aligned portions 21a and 21b spaced from one another to provide a gap within which is positioned means shown as an A.F. coil 22 for producing an alternating magnetic field H, having a substantialcomponent in a direction transverse to the earths magnetic field H, in the adjacent formations at a frequency corresponding to the Larmor precession frequency of the selected nuclei in the earths field H... Within the housing 20 there is also provided a detector coil 23 oriented with its axis in a direction transverse to that of the A. F. coil 22, and sub stantially' perpendicular to the earths field H,,, employed for detecting a condition of magnetic precessionproduced in the selected nuclei adjacent the logging instrument 14, in a manner to be described hereinafter.

wound in corresponding directions, so that the poles of the respective magnet portions 210 and 21b are in corresponding orientation (i.e.) with the north pole of one adjacent the south pole of the other. Advantageously, the cores 25a, 25b may be of laminated material to facilitate rapid switching of the D.C.

field H in response to correspondingly rapid current pulses applied thereto over the D.C. current supply circuit path 26.

The A. F. H, coil 22 is supplied with appropriate alternating current at the Larmor frequency of the selected nuclei in the earths magnetic field H, from a" power amplifier 27 inthe logging instrument 14 which, in tum,is supplied from the surface electronics 16 over the conductors 28 of the cable 15. The output of the detector coil 23 is connected to a preamp 29 for transmission of signals derived by the coil 23 over the appropriate conductive path 30 in the cable 15 to the surface electronics l6.

The directions of the magnetic fields H and H generated by the logging instrument 14 in an adjacent earth formation and of the earths field H, are represented by the vector diagram in FIG. 2. H represents the initial value of H This diagram will be better appreciated upon consideration of the method of operation of the logging instrument described hereinafter. v

In FIG. 3 the surface electronics 16 is shown withinthe dashed box as comprising a trigger generator 31 which serves as a master clock or timing source for generating appropriate timing pulses such as the saw-toothed trigger pulse shown graphically at the output thereof and which is coupled to the respective inputs of both a trigger andpulse generator 32 and a polarizing pulse generator 33. The polarizing pulse generator 33 provides an output signal inthe form of a square wave of predetermined duration, as shown, for controlling the output of a D.C. power source 34 coupled thereto which, in turn, provides output current over the circuit path 26 in the logging cable 15 to the coils 24a and 24b of, the D.C. magnetic field producing electro magnet 21a, 21b inthe logging instrument 14 only during predetermined successive time intervals for operation of the logging system as described herein.

The trigger and pulse generator 32 operates to provide two separate outputs, the first of which is a trigger pulse, as shown, which is supplied simultaneously to both the trigger of an oscilloscope, identified as C R O 44, and to agate with variable delay circuit 35 which, in turn, is coupled to a Boxcar integrator 36 which is part of the signal detection equipment which, in turn, is connected to the recorder 19.

The second signal output path from the trigger and pulse generator 32 provides a square wave gating signal, as shown, which is coupled to the input of a gated amplifier 37 which, in turn, serves to control the output of an audio oscillator 38, shown as a 2KC oscillator having its output continuously fed to the gated amplifier 37 which serves, in eflect, as an electronic switch to gate the output of the audio oscillator 38 in accordance with the square wave trigger pulse supplied to the gated amplifier 37, with the result that the gated amplifier 37 provides an output in the form of bursts of the audio oscillator output frequency, the duration and time separation of which are determined by the square wave gate signal from the trigger pulse generator 32. The output of the gated amplifier 37 is supplied to a class C amplifier 39 which, in turn, has its output coupled through an impedance matching network 40 to the signal transmission path 28 of the cable 15 which, in turn, is coupled to the amplifier 27 for supplying the prescribed A.C. current to the magnetic field producing coil 22 in the logging instrument 14.

It will be seen from the above that the supply of current to the D.C. polarizing magnet of the logging instrument and the supply of A.C. current to the A.F. coil 22 of the logging instrument 14, are both dependent upon gating signals provided by the trigger generator 31, which controls the Polarizing Pulse Generator 33 which, in turn, determines the operating schedule of the polarizing D.C. magnet 21a, 21b. Trigger generator 31 also controls the trigger and pulse generator 32 for determining the operating schedule of the AC. field producing coil 22, so that the operating schedules of the polarizing magnet and the AC. field producing coil are coordinated in a manner to be described hereinafter. In addition to the above, it is seen that the operation of the signal detection system is also correlated by the trigger generator 31 and the trigger and pulse generator 32.

The detector coil 23 is coupled through the preamp 29 to the transmission path 30 in the cable 15, thence to a signal amplifier 41 in the surface electronics 16, the output of which is supplied to a phase coherent mixer 42 (also known as a phase coherent detector) along with a CW reference signal, as shown, which is derived through a phase shifter and limiter amplifier 43 from the audio oscillator 38. The mixer contains circuit elements to filter the output to give a pass-band at the difference frequency between the outputs of circuits 41 and 43 and to eliminate the sum frequency. Thus, the reference signal supplied to the phase coherent detector 42 is provided in the desired frequency and phase relationship with the AF. or AC. field produced by the coil 22 and applied to the material of an earth formation adjacent the instrument 14.

The output of the phase coherent mixer 42 is coupled to a signal display device, shown as a cathode ray oscilloscope 44, which serves to provide a means for monitoring the operation of the system and for both visual and photographic observation of the signals displayed therein. The output of the phase coherent mixer 42 is also shown supplied to a signal enchancement system, shown as a boxcar integrator 36, which is operated in appropriate phase relationship with the precessing nuclei detected by the coil 23 as determined by the trigger pulse supplied thereto from the trigger and pulse generator 32 through the gate with variable delay circuit 35, which provides gating pulses of appropriately timed duration and spacing as shown. The output of the boxcar integrator 36, the operation of which will be described in detail hereinafter, is supplied to the recorder 17 which includes means for recording the detected nuclear magnetic resonance signal in correlation with a signal supplied over the circuit path 19 from the positioning means 18 showing the position of the logging instrument in the borehole where each nuclear precession measurement is made.

The apparatus illustrated in FIGS. 1 through 3 may be employed to conduct a log of the earth formations along a borehole by moving the instrument continuously at an appropriate logging speed through the borehole during the logging operation or by stopping the logging instrument at selected levels along the borehole as the logging signal is obtained.

The technique for conducting a log of the earth formations along a borehole may be better appreciated by reference to FIG. 2 wherein there is illustrated the orientations of the electro-magnet 21a, 21b, for producing the polarizing field H,, and of the coil 22 for producing the transverse alternating magnetic field H,, as well as the orientation of the detector coil 23 in the logging instrument, shown at right angles to the H, coil 22. Adjacent the logging instrument 14 there is shown a vector diagram of the magnetic fields of interest, wherein the X direction represents magnetic north, Y is magnetic west and the earths field H,,, at each sample point in the formation, is assumed to have a dip angle of 60, which is a representative value for much of the oil country in the United States of America. The desired strong polarizing field H,, is created with a substantial component perpendicular to the earth's field H,, at the sample point in the formation by the DC. polarizing magnet 21 a, 21b, as represented by the H,,, vector vertically directed at the sample point. The alternating field H, is produced with a substantial component perpendicular to the earths field H,. at the sample point by the H, coil which is sub stantially coplanar with the H polarizing magnet 21a, 21b. The detector coil 23 is oriented perpendicular to the H, coil 22 with its axis along the Y direction so that as the magnetic polarization M at the sample point rotates with a significant component in a plane perpendicular to the earth's field H, the lines of magnetic flux cut the detector coil 23 and induce a signal therein. The detector coil 23 is vertically elongated to provide greater effective length for cutting lines of flux. The DC. polarizing magnet 21a, 21b provides a magnetic flux at the sample point with an H, vector directed vertically.

Although the symbol H, has heretofore been used herein largely as a name for the alternating magnetic field, it will be understood by those skilled in the art that in all instances hereafter where the quantitative value of the alternating magnetic field is clearly intended (e.g. in formulas containing H,) it is intended that H, shall mean the magnitude of the component of the alternating magnetic field which is perpendicular to the unidirectional field about which precession takes place, which usually herein is the earth's field, H,..

The present invention involves a method of magnetic resonance analysis in the earths field H by achieving the condition of magnetic resonance through the steps of l) polarizing the sample with a strong field H,, in some direction having a substantial component at right angles to the earths field H,. (e.g. the Z direction); (2) applying in a direction having a substantial component at right angles to the earth's field H,. (e.g. the X direction) an alternating magnetic field H, at the resonance frequency H,,/2 rr and (3) turning off the polarizing field H,, adiabatically in a time Tummy given by the relationship:

T, T 1/-yH,; where T, is the thermal or spin-lattice relaxation time and H, is the strength of the alternating magnetic field at the sample. Typical values for protons in oil are: T, msec, H, E 5.0 oersteds, and ll'yH, =0.05 msec.

Under these circumstances the polarizing field H,, changes from a large value, H,,,- in the Z direction to a smaller value H,,, H,,, in the direction of H,,, and the magnetic polarization M precesses about the direction of the polarizing field H,, all the while as it changes from H,,, to H,,,= H,., ending in a condition of precession at the resonance frequency about the direction of the homogenous earths field H,,. This precessing condition can then be observed by means of the detector coil 23 so oriented that lines of magnetic flux emanating from the precessing polarization M cut the turns of the detector coil, e.g. by virtue of the detector coil orientation having its turns lying in the X-Z plane. The detector coil need not have its center at the origin of the XYZ axis although this would b preferable, if possible.

Applicant recognizes that although the above-described approach to magnetic resonance about the earths field H,. should ideally have the direction of H,,, H,, and H,, mutually perpendicular, such is not essential and in general is not practical. The mutually perpendicular condition is worthy for consideration for purposes of simplicity of illustration and understanding. In general, H,,, may be in any direction with respect to H,,, and H, need only have a component perpendicular to H, although the more nearly perpendicular the better. The detector coil is preferably oriented with its axis in a plane perpendicular to H,,; thus the detector coil may be the same coil as that which produces the H, field or a separate coil as is well known in nuclear magnetic resonance apparatus of the crossed-coil type, i.e., with axis not only perpendicular (or substantially perpendicular) to H, but also perpendicular to the axis of the H, coil, and as is shown in the embodiments FIGS. 2 and 3.

In order to clarify the relationship between the various factors in the approach to magnetic resonance, according to the present invention, it is helpful to view these factors from a point of view within a rotating frame of reference, rotating at the frequency of the rotating magnetic field H,. Thus instead of viewing them from the fixed frame of reference designated by the coordinates x, y z, we shall assume that we are located in, and viewing from, the rotating frame of reference designated by the coordinates x,,, y 1 Now let us assume further that the rotating frame has the same origin as the fixed frame, and also that z,, is aligned along z, and therefore that the x y plane is identical with the xy plane. Also we shall choose the direction of x,, to be the same as the direction of H,. Under these conditions it will be seen that all can now be represented in a 2-dimensional picture, since all factors lie in the x z plane, which itself rotates as seen in the fixed frame of reference. It will be remembered that in order to visualize what is actually happening in the fixed frame of reference one must restore the effect of rotation about the z axis.

When viewing the essential factors in the rotating frame of reference one finds that there is an effective field vector H, which is the vector sum of a first vector, H,, and a second vector whose value is obtained thus: H diminished by the value H which H,, has at resonance. Thus H, is a vector having magnitude [(H,,H +H, and having direction given by 0 arc tan (H,/H,, H The derivation of these relationships is given in Principles of Magnetic Resonance, by C. P. Slichter, Harper and Row (I963), page 19. Now the magnetic polarization M lines up with H, and continues to stay lined up with it assuming sufficient timeis allowed after each infinitesimal change in 0 to permit realignment. Therefore, starting with field,H, rotating at a constant frequency and with H,,, somewhat greater than H,,,, we approach magnetic resonance.

Graphically this is simply done by laying out H, along the x axis, then H H along the z axis, taking the vector sum graphically to get H and then laying out the magnetic polarization M along the direction of H Note that for the magnetic resonance condition, with 0 90, and H,.,,= H,, the magnetic polarization M would be found to be rotating in the xy plane if we return to viewing in the fixed frame of reference.

Adiabatic Variation of H permits M to follow H,

In the preceding paragraph an important assumption is made at one point; it is indicated by the underlined words: assuming sufficient time is allowed aftereach infinitesimal change in 0 to permit realignment." As shown in the above cited book by C. P. Slichter, pp. 2 l-22, this assumption is valid whenever the angular rate of turning of the field is small compared tothe product of 'y and the magnitude of the effective field. Stated interms of reciprocals, and using H, as the field which is turning, the assumption is valid whenever the time to turn H through one radian of angle is large compared to l/(-y H Noting that the magnitude of H... at the moment of passing through resonance is equalto H,, and using in place of the time to turn' H through one radian the more convenient term T defined as the time taken to efi'ect the entire change from H to H the above assumption may be said to be valid, and the criterion for what is called adiabatic variation" of the field is satisfied, when rhmugr- /(7 "1)- In actual practice this is not a difficult requirement, since one is free to choose H, large enough to satisfy it. In practice, in

order to make the mathematical analysis simple, we also require that H, be small compared to the static magnetic field at which resonance occurs, (in this case H since H is zero).

The significance of the preceding paragraph is this. When the conditions of adiabatic variation are met, as is assumed here, we can decrease the magnitude of H from some high value H along the z axis to final value H along the z axis equal to the value of H for which the frequency of the rotating field H, is right for magnetic resonance, gradually enough that M is, for all practical purposes, continuously aligned with H,,,in the x z plane.

Now it will be seen, further, that if the conditions for adiabatic variation are preserved, there is no need to keep H constant in direction (along the z axis) while allowing its magnitude to decrease. While H decreases in magnitude it might also be allowed to change in direction, and the final result would again be that M is maintained in alignment with H, provided the conditions for adiabatic variation are still met.

The significance of the above paragraph is as follows. When the conditions of adiabatic variation are met, as is assumed here, we can decrease the magnitude of H from some high value H along the z axis to a final value H along any axis, where H, is equal to the value of H for which the frequency of the rotating magnetic field H, is right for magnetic resonance, gradually enough that M is, for all practical purposes, continuously aligned with H, wherever it may be directed. In the special case where the direction of H0, corresponds to the direction of the y axis in the fixed frame of reference, we have M aligned with H, and, in the fixed frame of reference, rotating in the xz plane.

Limitations Upon Rate of Change of H In the above paragraphs it will be recalled that one limitation is placed on the rate of change of H viz., that it must change slowly enough that An upper limit may also be placed on T as follows. As H shifts from large H to small H,,,-, the magnetic polarization M will tend to decrease from a large value corresponding to H to a small value corresponding to H However the rate at which M can be allowed to change is limited by the spin-lattice relaxation time, T,, so that if T is greater than T,, M will be able to diminish along with H but if T is less than T,, it will not be able to follow the changes in H Now if we wish M to remain as large as possible during the change, it is clear that T must be made less than T,. Summing up, we can express the operating range for Tammy as follows:

T1 ehmux? HI if M is both to follow H in direction and to remain as large as possible. For simplicity in mathematical analysis we also require H, H when H is zero.

Method ofObservation Measuring r, and T;

To illustrate how observations may be made, we shall employ the embodiment of FIGS. 1-3. FIGS. 4a through 4d show the time duration of application of field H and H, and the resulting behavior of the magnetic polarization'M and the detector signal.

Let us assume that the embodiment of FIG. 2 is employed. Current is assumed to have been flowing in the electromagnet 21a, 2112 at a steady value for a time long compared to the I thermal, or spin-lattice, relaxation time,- T,,of the sought for nuclear species, with the result that the magnetic polarization M is at full value, M at a sample point in the formation and is aligned with the vector H,,, as shown in the vector design in FIG. 2. The quantities H and M are at this moment equal to H and M as shown in FIG. 4a and FIG. 40, respectively, where the instant of time is indicated by the numeral 1. Next the alternating field H,, oscillating at the Larmor frequency for the nuclear species in the earth's field H,, is applied in the direction shown in FIG. 2; this is done at the instant of time shown by the numeral 2 in FIG. 4b. Strictly, time 2 need only be any time prior to time 3. Shortly thereafter the electromagnet current is cut off at the instant 3 in FIG. 4a, and H is caused to die off at the controlled rate specified herein in the section on Limitations upon Rate of Change of H to the point that there is effectively no applied field H remaining, and the only unidirectional field left is the earth's field I'L, directed as shown in FIG. 2. At this moment, indicated by the numeral 4 in FIG. 40, the magnetic polarization M begins to precess at right angles to H... (Prior to the instant of time4, and in fact ever since the instant 2, M has been aligned along the effective field, H, it is only at the instant 4 when H, has both diminished to the value of H, and that M has reached directional alignment with H, that M is precessing at right angles to H...) As shown in FIG. 4c, this magnetic polarization M has heretofore been at full value M and now it begins to decay with a time constant T which is the true spin-spin relaxation time, with dephasing of the nuclear magnetic moments being caused by spin-spin interactions only. The nuclear magnetic moments are driven in precession by the H field and are locked in, so that other dephasing factors, such for example as inhomogeneity in the earth's field, cannot take effect. Strictly what is being observed is T H the true T in the earth field. In all practical cases of liquids this will also be T,(He). the spin-lattice relaxation time in the earths field. After H has been on, locking the magnetic polarization M in driven precession, for a time T lasting from the instant 2 to the instant 5 in FIG. 4b, it is abruptly turned off, and M thereupon enters into free precession and decays with relaxation time T Time 5 need only be any time after time 4, but the sooner after time 4 the better since the polarization M is diminishing with time. This relaxation time includes the effect of the earths field inhomogeneity as well as the other dephasing factors. The two difierent relaxation times are shown in FIG. 4c.

In order to make measurements which will yield these relaxation times, observe what the detector signal has been doing all the while. The detector coil of FIG. 2 has an EMF induced in it at the precessional frequency about H,.. FIG. 4d shows that from time 2 to time 5 the amplifier into which the detector signal is fed is blocked by the H, leakage field in a typical case. Between instants 5 and 6 the amplifier is making a short recovery, and thereafter the output of the combined detector coil and detector circuit (including amplifier) puts out a faithful reproduction of the magnitude of the decaying M vector shown above in FIG. 40. Clearly it is possible to measure T simply by observing this decay directly, as on an oscilloscope for example. Alternatively we can make a series of independent measurements of the magnitude of the decaying M vector at different times subsequent to the instant 6 and thus plot out the decay with time. It will be further explained below how such measurements can be carried out using the herein described apparatus. In order to measure T however, it is necessary to make a series of observations, using different length T and to measure the initial magnitudes of the detector signal (as at the instant 6) for each observation in the series. Each such observation gives one point on the T curve, and thus the curve can be constructed as shown in FIG. 4e. In every case where measurements of a decaying quantity are to be plotted vs. time, it is understood that the relaxation time can be obtained from the curve in well known ways. For example, for an exponential decay, one plots the logarithm of the decaying quantity vs. time and obtains the relaxation time from the slope of the straight line curve.

This invention is particularly well suited to use in petroleum well logging, and the description herein is directed primarily to that use, wherein the problem is to determine, by means of an instrument lowered down a borehole, whether there are hydrogen atoms present to a significant degree in the formations surrounding the borehole and also whether those hydrogen atoms are indicative of the presence of water or of hydrocarbons.

Although the invention is particularly well suited to use in petroleum well logging, it is not limited to that use. The well logging problem may be regarded as a special case of the more general problem where one wishes to measure the properties of a material which surrounds the measuring instrument, and the present invention is capable of making measurements of the properties of materials which surround the measuring instrument, whether they be earth formation materials as in well logging or, for example, the moisture content of a mass of material (grain, tobacco, soil sample) into which the measuring instrument is inserted.

Furthermore the invention is not limited to measurements on protons (i.e. nuclei of ordinary hydrogen), although the following description does dwell principally on measurements with protons. Protons are particularly important in petroleum well logging, since they are the indicator of the presence of hydrogenous material and further since their characteristic modes of behavior in water andin hydrocarbons are the key to whether there is water or hydrocarbon present. However, there are other chemical elements than hydrogen, the presence of which is important to know, whether for their own value or for stratigraphic correlation. This information may be desired as a guide for example to the presence of minerals such as fluorspar, phosphates, bauxite, borax, and spodumene. These minerals may be detected, using the present invention, by tuning to the following isotopes respectively, which respond to nuclear magnetism detection: fluorine-l9, phosphorus-31, aluminum-27, boron-l l, and lithium-7. The present invention is particularly well suited to making observations on such elements, and the reason is as follows. The present invention is capable of achieving a very high effective signal to noise ratio, with the result that many chemical elements generally considered too difficult to detect by nuclear magnetic resonance under adverse instrument conditions, such as in a borehole, can now be detected. Hydrogen has the highest nuclear gyromagnetic ratio of any atom, and the hydrogen atoms in water or inoil are present in high concentration, with the result that hydrogen has in the prior art been a preferred element to observe. Other elements, such as those named above, have lower gyromagnetic rations and are present generally at lower concentrations, and this means a poorer signal to noise ratio. However, the present invention makes it feasible to work with such elements other than hydrogen.

Measurements with Elements Other than Hydrogen As stated above, the invention is not limited to measurements on hydrogen nuclei but is also applicable to nuclei of other chemical elements, such as fluorine, phosphorus, aluminum, boron, and lithium for example. In principle the method is the same as described for hydrogen. However for the reasons given below in this same paragraph it would be advantageous to maintain a low unidirectional field H,, (as shown by dashed line in FIG. 4a) subsequent to the turnoff of the strong H and all during the periods of time in which the precession of M is taking place and measurements are being made. When prospecting for solids, e.g. minerals in the earth, we are dealing with materials which have inherently a greater line width than fluids, such as oil and gas. This line width for solid materials may be as much as several gauss, and it is as if there were now an inherent inhomogeneity built into the material under observation and therefore the homogeneity of the earths field is greater than we can take advantage of. Under these conditions there is no need to restrict ourselves to the low strength of the earths field for the precessional field, and we may as well enjoy the advantage of as high an applied unidirectional field as we can obtain within the limitation that its inhomogeneity shall not exceed the inherent inhomogeneity of the material under observation. Incidentally also it should be mentioned that because of the lower gyromagnetic ratio of these other elements as compared to hydrogen, their precession frequencies in any given unidirectional field are correspondingly lower. This presents the drawback that, while the 2 kilocycle per second precession of hydrogen in the earths field is not too difficult to make measurements with, the lower frequencies which would be found with the other elements would become too low for convenient measurements. The use of a low unidirectional field, greater than the earths field, for the precession field as explained above thus alleviates to some extent the problem of making measurements on elements other than hydrogen.

To illustrate how observations may be made upon elements other than hydrogen, we shall again employ the embodiment of FIG. 2 and shall discuss it in terms of FIG. 4a through 4d. The procedure is identical to'that given above under Method of Observation Measuring T and T up to the application of the H field. Then the alternating field H,, oscillating at the Larmor frequency for the nuclear species in the low unidirectional field H,,', is applied in the direction shown in FIG.2; this is done at the instant of time 2 in FIG. 4b.

Thereafter the procedure for this purpose is the same as that outlined in the above quoted section with the exception that when we cut off H, we let it drop to a value H,, rather than to zero and we now substitute low field H wherever H,, occurred before.

Measuring IUIIflh mm) The foregoing method is a slow-turn-off" (STO) method to give the value of true T at low field by a series of observations, each with different T and also the value of T at low field, e.g. by oscilloscope display. Moreover as stated above, in all practical cases of liquids T at low field is equal to T, at low field.

However it is known that T, in general has different values in high and in low fields. Consequently it is sometimes desirable to measure T, at high field. There follows a description of a slow-tum-off"c method for making such a measurement.

To measure T, in high field we make a series of observations, each observation being similar to that in FIGS. 4a through FIG. 4d, but with the polarizing field H on for less than sufficient time to fully polarize the magnetic moments, i.e. for times T,. of the order of and less than T,, as shown in FIGS. 5a,b, c, and d. In the observation beginning as illustrated in FIG. 50, H is applied for the short interval T starting at the instant of time I and ending at instant 3, it is to be understood that the ending at 3 is a slow-turn-off" as defined above; again as in FIG. 4b the alternating field H, is applied from the time 2, (prior to 3) until the time 5 (subsequent to time 3 plus T,.,,,,,,,,,.), as illustrated in FIG. 52'; again we get the behavior of the magnetic polarization M and the detector output as illustratedin FIGS. 40 and 4d, and here it is illustrated in FIGS. 5f and 5g for the case of M not fully polarized. The value of M reaches the maximum value identified by M in FIG. 5f, and the Maximum Detector Output, Free Precession is as shown at the time identified by the numeral 6 in FIG. 5g. This gives us one point for the curve plotted in FIG. 5h, showing Maximum Detector Output, Free Precession" as a function of T A second observation is made, beginning as illustrated in FIG. 5b, wherein H,, is applied for the time interval T starting at time I and ending at time 3; again H, is applied from time 2 until time 5; again we get M reaching the maximum value M in FIGfSf, and Maximum Detector Output, Free Precession isas shown at time 6 in FIG. 5g. Thus a second point is obtained for the curve of FIG. 5h. This process is continued as indicated in- FIG. 5 until the whole curve of FIG. 5h is obtained. This curve is known to have the form, a Constant X l eT /T,), and consequently the value of T can readily be deduced from it in well known ways.

Measuring Tuluu' field) Although in general it is not necessary to measure T once we know T since T T, for low fields, it is possible also to measure T, at any low field H In order to get T we observe the efi'ect of allowing the magnetization to relax for different durations T, in a low field, without precessing, before ininitiating the H, field and making the observation of the Maximum Detector Output." The fact that the relaxation is proceeding without any precession going on at the same time assures that the effect is pure T,-relaxation, i.e. spin-lattice relaxation, unmixed with spin-spin and field inhomogeneity factors. The fact that the relaxation is allowed to proceed in the presence of a low field H rather than the strong field H,,,, assures that the spin-lattice relaxation time observed is that which is characteristic of the low field phenomenon. not the high field phenomenon. The procedure for determining T is illustrated in FIG. 6. One observation, yielding one point on the final curve, is as follows: H,,, is applied from time I to time 3, an interval of time long compared to T, as shown in FIG. 6a. At time 3 H is reduced from the strong field value H,,, to the low field value H,, at which value it remains for a time T,,, which is the interval from time 3 to time 4 (Note that the turn-off at time 3 need not be a slow turn-off as defined above unless the direction of M undergoes a change during the turn-off; for practical purposes then the tum-off at time 3 need not bea slow turn-off as long as H,, is much greater than H,,). At time 2, intermediate time 3 and time 4, the altemating field H, is applied briefly until time 5, subsequent to time 4 plus T as shown in FIG. 6b. The magnetic polarization M, having been at full value M until the tum-off at time 3, experiences relaxation (without any precession) in the low field H,, remaining after the turn-off of H g. This relaxation is shown in FIG. 6e. At time 4, H is further reduced by a slow-tum-off as defined above from H,, to zero, thus leaving the earths field H, as the only unidirectional field remaining after time 4. As in the arrangements depicted in both FIGS. 4 and 5 the time 4 marks the beginning of driven precession about H,,, and again as in the preceding arrangements the time 5 marks the beginning of free precession about H,,. The detector receives a signal from the precessing magnetic moments beginning at time 2 and continuing until the precession decays after time 5, but it is only the latter part of the signal which is seen in the detector output, since the amplifier is blocked from time 2 to time 5 as shown in FIG. 6f. At

time 5 the amplifier makes a short recovery, and subsequent to time 6 it gives a faithful measure of the decaying free precession signal. At the time 6 the observation is made, and this is the "Maximum Detector Output, Free Precession." which is plotted as indicated by the numeral 6 against the appropriate value of T,, in FIG. 6i. The second observation, yielding a second point for the curve, differs only in that H,, remains on for a time T,, (greater than T which is the interval from time 3 to time 4 (later than time 4), and H, is pulsed on for the same duration as before but beginning at time 2' intermediate time 3 and time 4', as shown in FIG. 60. The detector output is correspondingly delayed and has a correspondingly different value for the. maximum output under free precession as shown at time 6' in FIG. 6g. This kind of observation, varying T and getting a corresponding value for the Maximum Detector Output, Free Prec'ession," can be made as many times as desired, and the points so obtained define the curve for relaxation time T, (low field, H asshown in FIG. 61'.

It is also possible to determine T, for the limiting case of the earths field, i.e. H equal to zero. This measurement will be seen to be similar to that shown in FIG. 6 for low field H,,. The difference is that at time 3, H is reduced by slow-turn-off to zero, thus allowing the magnetic polarization M to align itself with the earths field H,,. Upon reaching this alignment M is practically at full value M and now begins to experience relaxation (without precession) in the low field H,,, as shown in FIG. 6e. To obtain T, under these circumstances one permits the relaxation to proceed for a first measured time T and measures M, then one starts again and permits the relaxation to proceed for a second measured T and measures M again, etc. The step of measuring M consists, as shown in FIG. 6a, of first (using a slow turn-on) applying perpendicular to H,, at time 1A, delayed T after time 3, a polarizing field of reduced magnitude H,,, which is large compared to H, and to H,, to get the M vector again into a plane at right angles with the earths field H,,, then at time 2 applying the alternating field H,, oscillating at the Larmor frequency for the nuclear species in the earths field H,,, then at time 4 executing a slow-tum-off of the polarizing field H and allowing the M vector to enter into driven precession about H... and then at time 5 removing the H, field and allowing the M vector to enter into free precession about H,, and finally making a measurement of the magnitude of the M vector as in the previously described methods. A second, third, etc. measurement of M can then be made similarly, using different delay times T T etc. between the moments of time designated 3 and 1A, 3 and 1A, etc. It will be seen that the M vector is decaying according to T, for earths field only in these intervals T T T etc., for it is only in these intervals that the M vector is aligned with and subjected to the pure earths field H,, unmixed with other fields. At time 1A, or 1A, or 1A", the decay of M in the earths field is arrested, whereupon by use of H and H the M vector is placed into free precession in the characteristic manner of this invention, thus permitting measurements to be made of M corresponding to the several different delay times, T T T etc. as shown in FIGS. 6 6g, 6h, etc., and values to be plotted as shown in FIG. 6i.

Apparatus The above described methods can be carried out with the apparatus of FIGS. 1-3 which show a circuit for the flow of controlled DC power to the H coils, the flow of controlled audio frequency power to the H coils, and the output of modulated audio frequency signal from the detector coil to the detector channel. In the detector channel are shown blocks of apparatus, described above, for performing phase coherent detection, followed by boxcar integration of the signal, and final presentation of the output on the recorder. The detailed operations of this detector channel have not been described above and will be treated hereinafter. The combined phase coherent detection and boxcar integration is capable of substantial improvement in the signal to noise ratio, as will be explained below.

The reason that audio frequency equipment is specified is that the earths field having a value of approximately 0.5 oersted, the Larmor frequency for hydrogen nuclei (the most likely species to observe in nuclear magnetism logging) is approximately 2 kilocycles per second, which is in the audio frequency range. The adjustment of the audio frequency to the correct value for H, is discussed below.

First, to illustrate how any of the operations above described may be carried out, we shall describe the use of the apparatus of FIGS. 1-3 for carrying out the method of FIG. 4. However it will be seen that by choice of appropriately different sequences, the same apparatus can be used to carry out the methods of FIGS. 5,6, and 6' also.

In the method of FIG. 4 the H field is applied from time 1 to time 3. In FIG. 3 this is effected by the arrival of the trigger signal from the trigger generator 31 to activate the polarizing pulse generator 33, which feeds the DC power supply 34 for the H polarizing field coils. The duration of the polarizing field pulse, i.e. from 1 to 3, is controllable at the polarizing pulse generator 33. The cut-off of the polarizing pulse at time 3 is controllable by a shaping circuit, consisting of an integrator followed by a clipping circuit, so that the desired slowturn-off is obtained in accordance with the procedure explained hereinbefore. Concurrently with the initiating of the polarizing pulse, the same trigger signal enters the trigger and pulse generator 32. At a controlled time 2 prior to 3, the gate pulse lasting from 2 to is initiated and sent from the trigger and pulse generator 32 to the gated amplifier 37. A trigger pulse at time 5 is also sent from the trigger and pulse generator 32 to the gate with variable delay 35. The gate pulse (lasting from 2 to 5) passes to the class C amplifier 39 a gated portion of the continuous wave (CW) output from the audio oscillator 38. This gated audio pulse is passed through the impedance matching network 40 and the amplifier 27 to the H coil 22 of the nuclear magnetism logging apparatus and supplies the alternating H field pulse shown in FIG. 4b. The output from the detector coil 23 of FIG. 3 is fed through the pre-amplifier 29 to the amplifier 41 and thence to the mixer 42, yielding the mixed signal having for its trailing edge the free precession decay envelope formerly depicted in FIG. 4d, e.g. beginning at time 6. This output signal of circuit 41 is shown in FIG. 3 as the Nuclear Magnetism Signal," where it is depicted not as having the smooth, pure exponential shape of FIG. 4d but as having a distorted shape due to noise.

The nuclear magnetism signal is fed into the phase coherent mixer 42, where it is added to the CW reference signal, of the same frequency, which reference signal has originated in the audio oscillator 38. The CW reference signal has been passed through a phase shifter and limiter amplifier 43 where the phase of the reference signal has been adjusted to be the same as the nuclear magnetism signal. The output from the phase coherent mixer 42 is the mixed signal as shown in FIG. 3. It lends itself well to measurements which permit averaging out the noise component as will be explained further below. This mixed signal is fed into the boxcar integrator 36, which measures the signal in a preselected time interval as indicated in FIG. 3 between the times 7 and 8 for example. This interval is controllable as to duration by adjustments in the boxcar integrator 36 and as to time of initiation, i.e. the delay from time 5 to time 7, by adjustments in the gate with variable delay 35. The boxcar integrator 36 not only measures the signal in the prescribed interval, but also performs this measurement in that same interval in a number of repeated observations of the nuclear magnetism signal, and then integrates or averages all the measurements. Thereby the random noise component of the observed signal averages out nearly to zero and the desired information component is retained. The final output with signal to noise ratio thus enhanced is fed to the recorder 17 as shown in FIG. 3.

It is to be noted that the instant of time 7 and the interval of time 7 to 8 can be so chosen that the final output so obtained and fed to the recorder 17 in FIG. 3 is a measure of the Maximum Detector Output, Free Precession" specified by the instant of time 6 in FIG. 4d et seq., or of any point subsequent to time 6 on the curve of Detector Output." It is also to be noted that the output of the phase coherent mixer 42 is shown fed to a cathode ray oscilloscope (CRO) 44 in FIG. 3. This permits visual or photographic observation and measurement of the relaxation time T as mentioned in the section headed Method of Observation Measuring T and T Thus, the apparatus of FIGS. 1-3 is seen to provide the measurements required for the methods depicted in FIGS. 4, 5, 6 and 6'.

The apparatus of FIG. 3a is substantially identical with that of FIG. 3 (corresponding elements are identified with similar reference numerals), except that the apparatus of FIG. 3a does not employ a phase coherent mixer, or any other non linear detector for that matter, between the output of the signal amplifier 41 and the signal enhancement system which is shown as Multichannel Digital Integrator 36a in FIG. 30. It will be appreciated that since the Phase Coherent Mixer is omitted in the embodiment of FIG. 3a, no CW reference signal is required to be derived from the Audio Oscillator 38, thus the Phase Shifter and Limiter Amplifier 43 is unnecessary and is likewise omitted.

In the apparatus of FIG. 3a the nuclear magnetism signal derived from the Signal Amplifier 41 is supplied directly to the Multichannel Digital Integrator 36a, which is apparatus of the type sometimes described as a computer of average transients.

Advantageously, both positive and negative half cycles of every successive cycle of the NMR signal are supplied to the Multichannel Digital Integrator 36a in order to provide a rate of data acquisition which optimizes the duty cycle of the signal enhancement system during the time interval that the nuclear magnetism signal is being measured. A separate channel is required for each one half cycle since the signal changes sign each half cycle.

When the magnetic resonance signal is derived from nuclear magnetic precession in the earths field where the characteristic Larmor precession frequency is about 2 kilocycles per second a millisecond signal sampling interval requires a 400 channel digital integrator to provide maximum duty cycle operation for the system of FIG. 3a. I

In the embodiment of FIG. 3 we are free from the requirement of using a channel for each one half cycle, since the number of channels is determined by the requirements of the operator to record details of the decay envelope. Thus, more channels may be necessary if the signal is thought to contain several decay rates, e.g. depending upon the number of components of interest, than if only one decay rate is present.

Among the advantages of both the FIG. 3 and 3a systems is large dynamic range. This is because we do not use a nonlinear detection system in either embodiment. We use a linear 

1. In a method of geophysical exploration in bore holes drilled into the earth the steps of: subjecting a region including earth material in situ to a relatively strong D.C. magnetic field, for polarizing susceptible nuclei in said material including selected nuclei therein; applying an alternating magnetic field in the same region including polarized nuclei therein in a direction transverse to the earth''s magnetic field and at a frequency corresponding to the Larmor precession frequency of the selected nuclei in the earth''s field; while said alternating field is being applied, terminating the D.C. polarizing field in said region including said material adiabatically within a predetermined time interval such that the D.C. field is terminated within a time interval less than the characteristic thermal relaxation time of the selected nuclei, thereby sweeping the selected nuclei onto driven resonance in the alternating field; then abruptly tErminating the alternating field in said region including said polarized nuclei so that the selected polarized nuclei precess freely about the earth''s magnetic field in said region; and, detecting a characteristic of the resulting free precession condition of the selected nuclei.
 2. A well logging system for geophysical exploration in bore holes drilled into the earth comprising a well logging instrument adapted to be passed through the bore hole, said instrument including means for subjecting a region including earth material in situ to a relatively strong D.C. magnetic field, for polarizing susceptible nuclei in said material including selected nuclei therein; means for applying an alternating magnetic field in the same region including polarized nuclei therein in a direction transverse to the earth''s magnetic field and at a frequency corresponding to the Larmor precession frequency of the selected nuclei in the earth''s field; said system including means for terminating the D.C. polarizing field in said region including said material adiabatically while said alternating field is being applied and within a predetermined time interval such that the D.C. field is terminated within a time interval less than the characteristic thermal relaxation time of the selected nuclei, thereby sweeping the selected nuclei onto driven resonance in the alternating field; said system further including means for then abruptly terminating the alternating field in said region including said polarized nuclei so that the selected polarized nuclei precess freely about the earth''s magnetic field in said region; and, means for detecting a characteristic of the resulting free precession condition of the selected nuclei.
 3. Apparatus as defined in claim 2 wherein the means for applying the alternating field in the region including said polarized nuclei includes: a source of alternating current oscillations coupled to a coil in said logging instrument; and wherein said means for detecting a characteristic of the resulting free precession condition of the selected nuclei further includes a phase coherent detector coupled to said source of oscillations for deriving therefrom a reference signal in timed correlation with the alternating field applied to said region.
 4. Apparatus as defined in claim 3 further comprising a transient integrating type signal enhancement system coupled to the output of said phase coherent detector.
 5. Apparatus as defined in claim 3 further comprising magnetometer apparatus including a magnetometer head associated with said well logging instrument for measuring the value of the earth''s magnetic field in said bore hole, said logging system further comprising means coupled to said magnetometer for controlling the frequency of said source of alternating current oscillations in response to the value of the earth''s field in the vicinity of said logging instrument so that said alternating magnetic field applied to the region under investigation is maintained substantially at the Larmor precession frequency of the selected nuclei in the earth''s field from place to place throughout the bore hole.
 6. Apparatus as defined in claim 5 wherein said magnetometer is of the magnetic resonance type and which includes a magnetometer oscillator whose output frequency is maintained substantially at the Larmor frequency for selected nuclei contained in a preselected sample of material positioned in said magnetometer head and subjected to the earth''s magnetic field in said bore hole.
 7. A well logging system for geophysical exploration in bore holes drilled into the earth comprising a well logging instrument adapted to be passed through the bore hole, said instrument including means for subjecting material to a relatively strong D.C. magnetic field for polarizing susceptible nuclei therein; means for applying an alternating magnetic field to said material including polarized nuclei therein in a direction transverse to the earth''s magnetic field and at a freqUency corresponding substantially to the Larmor precession frequency of the selected nuclei in the earth''s field; said system including means for terminating the D.C. polarizing field applied to said material adiabatically while said alternating field is being applied and within a time interval less than the characteristic thermal relaxation time of the selected nuclei, thereby sweeping the selected nuclei onto driven resonance in the alternating field; said system further including means for then abruptly terminating the alternating field applied to said material including said polarized nuclei so that the selected polarized nuclei precess freely about the earth''s magnetic field in said material; and means for detecting a characteristic of the resulting free precession condition of the selected nuclei.
 8. Apparatus as defined in claim 7 wherein said material is included in earth formations in situ along the borehole.
 9. Apparatus as defined in claim 7 wherein said material comprises a preselected sample contained within said logging instrument, and wherein means are provided for deriving a signal indicative of the precise value of the Larmor frequency of the nuclei in free precession, thereby providing a measure of the earth''s field applied to said sample in the logging instrument.
 10. Apparatus as defined in claim 2 wherein said means for detecting a characteristic of the resulting free precession condition of the selected nuclei includes a transient integrating signal enhancement system.
 11. Apparatus as defined in claim 4 wherein said transient integrating signal enhancement system includes an analog boxcar integrator.
 12. Apparatus as defined in claim 10 wherein said transient integrating signal enhancement system includes an analog boxcar integrator.
 13. Apparatus as defined in claim 4 wherein said transient integrating signal enhancement system includes a digital boxcar integrator.
 14. Apparatus as defined in claim 10 wherein said transient integrating signal enhancement system includes a digital boxcar integrator.
 15. Apparatus as defined in claim 4 wherein said transient integrating signal enhancement system includes a multi-channel computer of average transients.
 16. Apparatus as defined in claim 10 where said transient integrating signal enhancement system includes a multi-channel computer of average transients.
 17. A well logging system for geophysical exploration in boreholes drilled into the earth comprising a well logging instrument adapted to be passed through the borehole, said instrument including means for subjecting a region including earth material in situ to a relatively strong D.C. magnetic field, for polarizing susceptible nuclei in said material including selected nuclei therein; said system including means for terminating the D.C. polarizing field in said region including said material adiabatically within a predetermined time interval such that the D.C. field is terminated within a time interval less than the characteristic thermal relaxation time of the selected nuclei; means for applying an alternating field in the same region including polarized nuclei therein in a direction transverse to the earth''s magnetic field and at a frequency corresponding to the Larmor precession frequency of the selected nuclei in the earth''s field; said system further including means for then abruptly terminating the alternating field in said region including said polarized nuclei so that the selected polarized nuclei precess freely about the earth''s magnetic field in said region; and, means comprising a phase coherent mixer for detecting a characteristic of the resulting free precession condition of the selected nuclei, said phase coherent mixer including means for deriving a reference signal in timed correlation with the alternating field applied to said region.
 18. In a method of geophysical exploration in boreholes drilled into the earth the steps of: subjecting a region including earth material in situ to a relatively strong D.C. magnetic field, for polarizing susceptible nuclei in said material including selected nuclei therein; terminating the D.C. polarizing field in said region including said material adiabatically within a predetermined time interval such that the D.C. field is terminated within a time interval less than the characteristic thermal relaxation time of the selected nuclei; applying an alternating magnetic field in the same region including polarized nuclei therein in a direction transverse to the earth''s magnetic field and at a frequency corresponding to the Larmor precession frequency of the selected nuclei in the earth''s field; then abruptly terminating the alternating field in said region including said polarized nuclei so that the selected polarized nuclei precess freely about the earth''s magnetic field in said region; and, detecting a characteristic of the resulting free precession condition of the selected nuclei by deriving a signal corresponding to the detected characteristic by deriving a reference signal in timed correlation with the alternating field applied to said region and subjecting the derived signal to phase coherent mixing.
 19. Apparatus as defined in claim 17 wherein said means for detecting a characteristic of the resulting free precession condition of the selected nuclei includes a transient integrating signal enhancement system.
 20. Apparatus as defined in claim 19 wherein said transient integrating signal enhancement system includes an analog boxcar integrator.
 21. Apparatus as defined in claim 19 wherein said transient integrating signal enhancement system includes a digital boxcar integrator.
 22. Apparatus as defined in claim 19 wherein said transient integrating signal enhancement system includes a multi-channel computer of average transients.
 23. A well logging system as defined in claim 2 wherein the means for subjecting said region in situ to a relatively strong D.C. magnetic field includes first coil means, and wherein the means for detecting a characteristic of the resulting free precession condition includes second coil means.
 24. A well logging system as defined in claim 7 wherein the means for subjecting said material to a relatively strong D.C. magnetic field includes first coil means and wherein the means for detecting a characteristic of the resulting free precession condition includes second coil means.
 25. A well logging system as defined in claim 17 wherein the means for subjecting said region in situ to a relatively strong D.C. magnetic field includes first coil means, and wherein the means for detecting a characteristic of the resulting free precession condition includes second coil means.
 26. A well logging system as defined in claim 23 wherein the means for applying an alternating field in the same region includes third coil means.
 27. A well logging system as defined in claim 24 wherein the means for applying an alternating field to said material includes third coil means
 28. A well logging system as defined in claim 25 wherein the means for applying an alternating field to said material includes third coil means. 